The invention relates to a process for the generation of steam with a pressure of 3.0 to 6.0 bar and a temperature of 140.degree. to 165.degree. C. by evaporating and compressing heat-transfer liquids at a low temperature.
Whenever the term "bar" is used herein, it is to be understood as "bar absolute". Heat-transfer liquids normally obtain their heat capacity as a result of heat evolution in chemical processes and/or they are hot condensates.
An external heat supply is often required for chemical reaction or separation processes because certain reactions take place only at defined temperatures and/or in the presence of heat. The reaction products normally have to be cooled to ambient temperature and condensed. Whereas heat capacities with a temperature range of over 150.degree. C. are well suited for heat transfer because of the thermodynamic quality, heat capacities at temperatures of approximately 100.degree. C. can hardly be utilized and must therefore be dissipated by means of heat exchangers operated with air or cooling water.
Many chemical process plants have interconnected heating systems and normally they are integrated, i.e. most of their heating systems are coupled. Heat sources, such as product streams and utilities, that require cooling or product vapors that have to be condensed are exploited by transferring heat to boiler feed water in tubular heat exchangers in parallel or in series for steam generation. Part of the steam generated is used for driving turbines and other steam operated equipment and partly for heating the plant equipment. Condensates are thus obtained at different pressures and temperatures. Since the process units cannot constantly run at design load, i.e. part-load or even temporary shutdowns are inevitable, the steam system will have different pressure levels sufficient to supply slightly superheated steam at the specified temperature to distant plant equipment. For this reason, there are normally two levels: a medium-pressure level of approximately 15 to 25 bar and a low-pressure level of 3 to 6 bar. Medium-pressure steam is suited for a heating temperature of approximately 220.degree. C. or can be used as propellent steam for steam-jet ejectors or as driving steam for steam turbines. Low-pressure steam is chiefly suited for heating. It has a pressure of 3 to 6 bar and a temperature slightly above the saturated steam temperature. Thus, it can be safely piped to and used in distant plant equipment. If there is not sufficient low-pressure steam, it is necessary to withdraw steam from the medium-pressure system, to reduce it to the low pressure required and, if necessary, to inject condensate for desuperheating or saturating the steam. This method is uneconomical because precious steam of high thermodynamic quality is being lost.
Hot condensates of different pressures and condensation temperatures are also typical of chemical process plants. Steam condensates are re-evaporated and fed to the feed-water treatment unit. If the condensates have low pressure and temperature, they constitute a waste and are discharged into the sewer system at atmospheric pressure and approximately ambient temperature. This means prior depressurization and cooling with the aid of air and cooling water. The thermal potential thus cannot be recovered.
Processes are known in which the thermal potential of heat-transfer liquids, which is normally water and has a temperature of approximately 100.degree. C. is exploited by vacuum-evaporation, i.e. below 1 bar. The vapors obtained are mixed with propellent steam and constantly withdrawn by means of steam jet ejectors. Thus the vacuum can be maintained in the system. The pressure of the mixture is, for instance increased to 2 bar in the diffusor of the steam-jet ejector. This is an economical method of increasing the temperature of a certain part of the potential by approximately 25.degree. C. with the aid of high-pressure propellent steam.
If vacuum steam is obtained, the increase mentioned before is normally insufficient because the temperature rises only to approximately 110.degree. to 115.degree. C. In this case the steam must be used in the vicinity of the steam-generating equipment since it is almost impossible to pipe this steam to distant equipment and to use it as heating steam. Piping causes considerable pressure and temperature drops leaving only a hot condensate. Moreover, the temperature is too low in this case. If heat-transfer liquids with different pressures are obtained simultaneously, it is imperative to provide for different steam-jet ejectors, each designed for a certain pressure ratio. If the pressure ratio also varies due to intake pressure rise or drop, the steam ejector will exhibit an unstable and uneconomical performance.
Steam-jet ejectors of special design have proved to be uneconomical for boosting the pressure of over 2 bar because the necessary quantity of propellent steam is a multiple of the intake quantity, i.e. an excess of LP heating steam is obtained.